1. Technical Field
The present invention relates to signal processing devices such as image rejection mixer circuits which remove components of image signals generated when high frequency signals and local signals are mixed and down-converted into intermediate frequency signals (IF signals), and to a signal processing method.
2. Related Art
In an IF receiver in which an IF (Intermediate Frequency) is set to be relatively low, an IF filter can be formed of a semiconductor element, which provides an advantage that an outside SAW (surface acoustic wave) filter is not required.
However, setting the IF frequency of an IF receiver to be low means to set the frequency of local signals used for converting the frequency of RF (radio frequency) signals relatively near the frequency of the RF signals. Therefore, the frequency of an undesired signal (hereinafter referred to as an image signal) positioned symmetrical to a desired signal with reference to the frequency of the local signal on the frequency axis comes close to the frequency of the desired signal, whereby it becomes difficult to remove the image signal prior to a down-conversion.
Thus, down-conversion mixers must be provided with such a function to down-convert only desired signals and to serve as image rejection mixers for suppressing image signals.
FIG. 16 is a schematic diagram showing the configuration of a Hartley-type image rejection mixer as a typical image rejection mixer according to conventional art. The Hartley-type image rejection mixer shown in FIG. 16 is described in the U.S. Pat. No. 1,666,206 and has been well known.
The image rejection mixer shown in FIG. 16 is provided with input terminals 1, 4, 5, mixers 2, 3, a phase shifter 6 as a phase delaying means, an adder (adding means) 7, and an output terminal 34. The mixer is so configured as to convert the frequency of RF signals input from the input terminal 1 and output IF signals from the output terminal 34.
The operation of the Hartley-type image rejection mixer shown in FIG. 16 will be described below. FIG. 17 is a chart showing an example of frequency distribution of an input signal input into the Hartley-type image rejection mixer shown in FIG. 16. In FIG. 17, the vertical axis shows the frequency of an input signal and the horizontal axis shows magnitude of an input signal. To simplify the explanation, the input signal is assumed to include only two desired signals 14, 15 with frequency of ±ω1 respectively, and two image signals 16, 17 with frequency of ±ω2 respectively. Note that the frequency ω1 of the desired signals 14, the frequency ω2 of the image signal 16 and the frequency ωLD of the local signal have a relationship of (ω1+ω2)/2=ωLD.
The RF signal input into the input terminal 1 shown in FIG. 16 is distributed to two branches. The RF signal in one branch is mixed with the local signal cos(ωLD*t) (a first periodic signal) input into the input terminal 4 in the mixer 2, and the RF signal in the other branch is mixed with the local signal sin(ωLD*t) (a second periodic signal) input into the input terminal 5 in the mixer 3. As a result, the mixer 2 outputs the down-converted signal as an I-signal, and the mixer 3 outputs the down-converted signal as a Q-signal. Then, the Q-signal passes through the phase shifter so that the phase is delayed 90 degrees.
FIG. 18 shows the state of a down-conversion from the input signal into the I-signal. In FIG. 18, the corresponding relationships in the down-conversion of the desired signals are shown by the arrows of solid lines, and the corresponding relationships in the down-conversion of the image signals are shown by the arrows of dotted lines.
As shown in FIG. 18, in the mixer 2, the desired signals 14, 15 are down-converted, whereby down-converted desired signals 18, 19 are obtained. The frequencies of the down-converted desired signals 18, 19 are ±(ω1−ωLD).
Further, in the mixer 2, the image signals 16, 17 are down-converted, whereby down-converted image signals 20, 21 are obtained. As obvious from FIG. 18, the down-converted desired signals 18, 19 and the down-converted image signals 21, 20 appear so as to be superimposed around frequencies of ±(ω1−ωLD), respectively.
FIG. 19 shows a state of down-conversion from the input signal to the Q-signal. In FIG. 19, the corresponding relationships in the down-conversion of the desired signals are shown by the arrows of solid lines, and the corresponding relationships in the down-conversion of the image signals are shown by the arrows of dotted lines.
FIG. 19 shows the down-converted signals 22 to 25, which phases are delayed 90 degrees by the phase shifter 6.
In the mixer 3, the desired signals 14, 15 are down-converted, whereby down-converted desired signals 22, 23 are obtained, respectively. The frequencies of the down-converted desired signals 22, 23 are ±(ω1−ωLD).
Further, in the mixer 3, the image signals 16, 17 are down-converted, whereby down-converted image signals 24, 25 are obtained, respectively. The down-converted desired signals 22, 23, which are obtained by down-converting the desired signals, and the down-converted image signals 25, 24, which are obtained by down-converting the image signal, appear around frequencies of ±(ω1−ωLD), respectively. In this case, there is a phase difference of 90 degrees between the local signals (cos(ωLD*t), sin(ωLD*t); frequency ωLD) input into the mixers 2, 3. Further, due to the phase delay by the phase shifter (90-degree phase shifter) 6, in the case of the down-conversion shown in FIG. 19, the phases of the down-converted desired signals 22, 23 and those of the down-converted image signals are shifted 180 degrees.
In FIG. 19, the down-converted image signals 24, 25 shown downwardly represent the down-converted image signals with phase shifted of 180 degrees from the phases of the down-converted desired signals 22, 23.
When the down-converted I-signal and the Q-signal, shown in FIGS. 18 and 19, are added by the adder 7, each of the down-converted image signals 20 and 24, and 21 and 25 have the same amplitude with opposite polarity, as shown in FIGS. 18 and 19. Therefore, they offset each other, whereby only the down-converted desired signals 26, 27 with added amplitude of the down-converted desired signals 18, 19 and 22, 23 are obtained.
However, if there is gain deviation between the I-signal path from the mixer 2 to the adder 7 and the Q-signal path from the mixer 3 to the adder 7, the image signals do not offset completely at the adder output, whereby the image signals are not suppressed completely.
Further, if the 90-degree phase shift in the phase shifter 6 is not enough accurate, or if the phase difference between the signals with a frequency ωLD(cos(ωLD*t) and sin(ωLD*t)) input into the mixer 2 and the mixer 3 is shifted from 90 degrees, the image signals are not suppressed completely as well.
FIG. 21 shows the aforementioned phenomenon where amplitude differences arise between the down-converted image signals 18 and 24, and 19 and 25. Therefore, they do not offset each other by addition. Consequently, the down-converted image signals 28 and 29, are output from the adder 7. Thus, the output from the adder 7 includes the down-converted image signals 28, 29 in addition to the down-converted desired signals 26, 27, and the down-converted image signals 28, 29 are superimposed on the down-converted desired signals 26, 27.
In equipments such as radio ones, it is required to support a case where the intensity of the image signal is larger than that of the desired signal. In such a case, for example, the image rejection ratio of more than 50 dB is required for an image rejection mixer.
However, when considering the process fluctuations of transistors and/or passive elements constituting the image rejection mixer, device characteristic drift due to the temperature variation, and so on, the typical image rejection ratio of conventional image rejection mixers is up to 30 dB.
One of a conventional technique to enhance the image rejection ratio of image rejection mixers is to add a filter for removing the image signal in front of a down-conversion mixer.
However, in IF receivers with relatively low IF frequency as described above, the frequency of a desired signal and the frequency of an image signal are close. Thereby, a loss of the desired signal caused by the aforementioned image rejection filter becomes large, resulting in performance degradation of the IF receivers.
Further, in such an image rejection filter, the frequency band of the desired signal is in the RF region, whereby it is impossible to integrate it in a semiconductor chip. This causes problems in miniaturization and cost reduction.
Another conventional technique is a feedback control method so as to increase the image suppression ratio, where the mixers 2, 3 and the phase shifter 6 are tunable depending on the external control.
However, this method has difficulties in in-situ monitoring of characteristics in each components compensating operation under temperature variation, and accuracy required in components for feedback control which are newly added. These causes yield drops, cost increase, and lack of stability.
As described above, in conventional image rejection mixers, there is a problem that undesired image signals remain due to the circuit nature during signal processing, which are superimposed on the desired signals.
Further, the conventional technique for suppressing the image signals (image leakage) causes such shortcomings that the performance of the equipment deteriorates, the cost becomes higher, power consumption increases, and the suppressing function does not fully work in some cases.
An object of the present invention is to provide a signal processing method and an equipment which is capable of suppressing undesired image signals, which are generated in the signal processing and are superimposed on desired signals.